Various types of injection devices are available for assisting with the injection of a medicament into a patient (human or animal), and which are configured to receive a standard, pre-filled glass or plastic syringe tipped with an injection needle. These devices may have a dose setting mechanism and a main drive spring for driving a plunger into the syringe so as to expel the medicament out through the needle. Injection devices may comprise a further spring for driving the needle out of the device housing and into the patient's skin, prior to activation of the main drive spring to expel the medicament.
In order to maintain sterility prior to use, and to avoid “sticking” injuries, the pre-filled syringe is supplied to the injection device assembler with a rubber or plastic cap, known as a “sheath”, covering the needle. The sheath has an interior space for containing the needle, and a sealing end that abuts the adjacent end of the syringe barrel to seal that inner space.
Immediately prior to use, a user (e.g. healthcare professional or patient) must remove the sheath to uncover the needle. This is typically achieved using a sheath removal tool that is inserted by a user into the injecting end of the device. The tool comprises a set of sprung fingers that ride over and along the sheath as the tool is pushed into the device. The fingers then snap into the junction between the syringe end and the sheath. The user can then pull out the tool, bringing the sheath with it.
Particularly in the case of expensive medicaments, it is extremely important to minimise the failure rate of assembled injection devices. Considering the sheath removal solution outlined in the previous paragraphs, it may be difficult to achieve exactly the right degree of flexibility in the fingers to ensure that they can ride over the sheath whilst still providing sufficient force to close over the junction at the rear of the sheath.
GB 2438593 (Cilag), US 2006/0100588 A1 (Brunnberg et al), WO 2007/047200 A1 (Eli Lilly) and US 2006/0270986 A1 (Hommann et al) all describe devices for housing syringes and removing sheaths therefrom. US 2001/0031949 A1 (Asbaghi) is not concerned with sheath removal but with only with preventing accidental re-use of a syringe after injection and with prevention of “stick” injuries at that time. Disclosed is a guard, a guard body and a spring. The device passively covers and protects an exposed needle post-injection by locking the guard housing over the tip of needle. This is achieved via a linear slot and cooperating plug arrangement. The slot has a latching cut out between the slot ends and a locking cut out. The locking cut out is positioned at an injection end of the device and, after use, locks the guard to the guard body to ensure that the guard completely covers the needle tip. A spring is provided to drive the guard towards the injection end of the device relative to the guard body, but this motion towards the injection end is limited by the latching cut out. To remove the sheath, the syringe is held in place by the latching cut out and, once removed, the device is arranged such that the needle tip is partially exposed beyond the guard. After injection, the locking cut out serves to keep the needle tip covered by the guard.
WO 01/68164 A1 (International Technology Group) provides a safety syringe, comprising an outer tubular protective cover sleeve for an inner syringe body, to prevent a needle “stick” injury. GB 2 451 665 A (Cilag) describes an injection device with a housing with an exit aperture covered by a cap, a syringe carrier, a needle sheath and a locking component. In use, the lock operates in conjunction with the cap. Upon removal of the cap, the lock is automatically released. Until released, the lock the serves to prevent forward motion of the syringe, thereby preventing damage to the syringe prior to use. GB 2 451 662 A (also by Cilag) describes an auto-injector device with a trigger and locking mechanism for restraining and triggering a drive spring to, inter alia, prevent forward motion of a syringe carrier before triggering. A pivoting trigger switch is mounted on a housing, between the drive spring and the syringe carrier. Upon triggering, drive is transmitted to the syringe via two rods connected by a piston so as to cause a time delay.